1. Field of the Invention
This invention relates to a control method and a control circuit for a laser diode (LD) and, particularly, to a control method and a control circuit that can cope with a change in optical output intensity of LD due to a temperature change and an elapsed-time change, and further relates to an optical transmitter using the same.
2. Description of the Related Art
An optical transmitter used for optical communications is equipped with a light source to output light with an intensity according to a bias current and a modulation current, and a modulation circuit to generate the modulation current according to a transmission signal. A transmission signal (herein, a digital signal with a voltage corresponding to logical “1” or “0”) sent from an external device is differentially amplified and buffer-amplified, and one or both of its differentially amplified signals are applied to a laser diode, and, thereby, an optical transmission signal can be obtained that its optical output intensity changes between “1” level and “0” level according to “1” and “0” of the transmission signal.
In optical transmitters, LD control is required such that OMA (optical modulation amplitude) and extinction ratio are increased as much as possible so as to obtain good identification sensitivity. The OMA and extinction ratio are values relating to the identification sensitivity of optical signal. The OMA is a difference between the “1” level and the “0” level of optical output, and the extinction ratio is a ratio of between the “1” level and the “0” level of optical output.
However, as the transmission rate of optical signal is increased, if the extinction ratio is too big, the oscillation of LD cannot follow, causing deterioration in waveform and an increase in jitter of the optical transmission signal. Further, in case of high-speed and long-distance transmission using an existing single mode fiber, if the extinction ratio is too big, a transitional change (chirp) is generated in the oscillation wavelength of LD. This causes deterioration in waveform after the long-distance transmission depending on a dispersion characteristic of optical fiber, and the identification sensitivity deteriorates by contrast. Therefore, in optical transmitters for high-speed or long-distance transmission, accurate control of extinction ratio is required.
On the other hand, LD has a property that its oscillation threshold current and slope efficiency change depending on temperature change and elapsed time change. Since the change of oscillation threshold current causes a change in optical output intensity and the change of slope efficiency causes a change in OMA and extinction ratio, control of bias current and modulation current is needed.
FIG. 1 shows a relationship between applied current and optical output intensity of LD. As shown in FIG. 1, when DC current (applied current I) exceeds an oscillation threshold Ith, the LD emits light. As the applied current I increases, the optical output intensity P increases linearly to the applied current I. The slope η (η=optical output intensity P/applied current I) of the linear section is called slope efficiency of LD. As shown in FIG. 1, when a bias current Ib and a modulation current Im are applied to LD, a modulated optical output can be obtained. The values of Ib and Im are set to obtain a predetermined optical output and OMA or extinction ratio.
FIG. 2 shows a temperature change and an elapsed-time change (relationship between the applied current and the optical output intensity) in LD. As shown in FIG. 2, the oscillation threshold Ith and slope efficiency η change depending on the temperature change and elapsed time change. In view of FIG. 2, it is understood that, when Ith is increased to Ith′ and η is reduced to η′ due to the temperature change or elapsed-time change, the predetermined optical output and OMA or extinction ratio cannot be obtained unless, according to such a change, the bias current Ibis increased to Ib′ and the modulation current Im is increased to Im′.
In order to prevent the change of optical output intensity in LD, APC (automatic power control) is used that the bias current is subjected to a feedback while monitoring the optical output. Methods for preventing the change of OMA or extinction ratio are as follows.
(1) Surrounding temperature of LD is measured using a temperature sensor and its modulation current is controlled depending on the surrounding temperature (for example, Japanese patent application laid-open No.2002-111120, paragraphs [0030] to [0050]).
(2) OMA or extinction ratio is measured using a peak hold circuit etc. based on optical current of PD (photodiode) to monitor the optical output, and its modulation current is subjected to a feedback (for example, Japanese patent application laid-open No.2001-352125, paragraphs [0025] to [0038]).
(3) OMA or extinction ratio etc. is measured such that a RF signal with low frequency is superposed on bias current or modulation current and only the RF signal component is extracted from optical current of PD to monitor the optical output. Based upon this, its modulation current is subjected to a feedback (for example, Japanese patent application laid-open No.2003-169022, paragraphs [0018] to [0020]).
(4) Based on the variation of bias current and modulation current, a bias current or a modulation current is estimated that the OMA or extinction ratio can be kept constant. Based upon this, its modulation current is subjected to a feedforward. This method is based on a statistical prediction that the change of bias current and modulation current with the change of LD characteristic is generated in like manner whichever of temperature change or elapsed-time change causes that change. For example, the ratio of a variation in bias current and a variation in modulation current is controlled to be constant, and the ratio is set in manufacturing. Alternatively, data of bias current and modulation current are taken that the OMA or extinction ratio can be kept constant while changing the surrounding temperature in manufacturing, and it is controlled based on the data.
Method (1) can be most simply implemented and is in wide use. Method (2) can surely control the extinction ratio since it measures directly the extinction ratio. Method (3) allows the collection of a peak value of optical output since it uses the low frequency RF signal applicable to the response speed of PD.
However, the conventional LD control methods (1) to (4) each have problems as follows.
Method (1) cannot cope with an elapsed-time change of LD characteristic. Further, in method (1), since individual difference in temperature characteristic of LD is big, it is difficult to conduct the adjustment in manufacturing.
In method (2), when the transmission speed of signal increases such that it exceeds the response speed of PD to monitor the optical output, it becomes impossible to detect the peak value. Further, in method (2), it becomes difficult to downsize an optical transmitter equipped with such a LD control circuit since the external circuits are needed.
In method (3), since the test signal is additionally superposed on the transmission signal, there is a risk such as deterioration in optical waveform and an increase, oscillation etc. in jitter. Further, since the external circuits are needed, it is not suitable for a high-density package.
In method (4), the guarantee for elapsed-time change is obtain only within the statistical data. Further, due to dispersion in LD characteristic, adjustment in manufacturing needs a large load.